The present invention relates generally to the field of voltage regulation, and more particularly, to on-load tap changers operating as voltage regulation devices.
Conventionally, electricity is generated in large-scale power plants that are connected to a transmission grid. Electrical power is transmitted over a transmission system over long distances at very high voltages. At distribution substations the voltage is stepped down and power is supplied to different loads within a distribution grid. Voltage regulation in the distribution grid is typically achieved through voltage regulation devices such as on-load tap changing transformers or voltage regulators. Capacitor banks are also widely used in many utilities to support the voltage regulation in distribution grids, where voltage variations are mainly caused by slow variation of loads connected to the distribution system. With the growing penetration of intermittent renewable energy resources connected at distribution level, voltage variations in distribution grids are aggravated and becoming more frequent. This development requires more flexibility in network voltage regulation leading to an increased and more extensive utilization of voltage regulation devices in distribution grids.
Voltage regulation devices, such as on-load tap changing transformers, are used to provide regulated voltage to the output terminals. On-load tap changing transformers typically include at least one primary winding and at least one secondary winding. The primary and secondary windings include a plurality of turns. Input voltage is provided to the primary winding and the electric load is coupled to the secondary windings. Magnetic interaction between primary and secondary windings causes energy to be transferred from the primary winding to the secondary winding. Transformers convert the input voltage (Vin) at the primary windings to an output voltage (Vout) at the secondary windings based on a turns ratio (T2/T1) of the secondary winding turns (T2) versus primary winding turns (T1). The output voltage is computed based on equation 1:Vout=Vin×T2/T1  (1)
An on-load tap changing transformer has several connection points, so called “taps”, along at least one of its windings. With each of these tap positions a certain number of turns is selected. Since the output voltage of the on-load tap changing transformer is determined by the turns ratio of the primary windings versus the secondary windings, the output voltage can be varied by selecting different taps. On-load tap changers (OLTCs) are used to change the tap position of an on-load tap changing transformer while energized, i.e., under load.
Different mechanisms have been developed for OLTCs to change the turns ratio of the primary windings versus the secondary windings of on-load tap changing transformers. Several types of OLTCs, both mechanical and electronic, are available in the market. Mechanical OLTCs allow for in-service operation, but have demanding mechanical requirements. Each tap changing operation of mechanical tap changers leads to a certain amount of arcing between tap contacts and moving finger contacts. Arcing leads to slow deterioration of the transformer oil and accelerated wear-and-tear of mechanical contacts. The lifetime of a mechanical tap changer is hence limited by the number of tap changing operations. Conventional OLTCs have nevertheless a relatively long lifetime of 15-20 years. This is mainly due to the comparably low number of tap changing operations required to regulate the slow voltage variations due to loads. However, more frequent voltage fluctuations in distribution networks can be seen nowadays which are caused by the increasing share of distributed generation by means of renewable energy sources. Therefore, OLTCs are required to operate more frequently than before. This leads to much higher maintenance requirements and limited lifetime. Furthermore, mechanical OLTCs require current limiting inductors or resistors to limit the short-circuit current, which is present during a tap changing operation. Consequently, a need for cooling these current limiting devices may arise due to frequent tap changing occurrences.
The main drawback of mechanical on-load tap changers is unavoidable arcing between the tap contacts and the moving finger contacts when a tap is changed. Purely electronic on-load tap changers on the other hand do not have any moving mechanical contacts. Each tap contact is connected to the load through a solid-state electronic switch. The tap position is selected by switching on the corresponding electronic switch (i.e. conducting), while all other switches are switched off (i.e. not conducting). Changing from one tap position to the other is carried out by commutating the current from one electronic switch to the next. The current commutation is therefore achieved without arcing due to the typically very fast switching capabilities of solid-state switches. Although electronic OLTCs are highly flexible and can operate arc-free and would therefore substantially reduce maintenance requirements as compared to mechanical OLTCs, they also have certain disadvantages. The main drawback is the high cost of electronic switches. Since an electronic switch is required for each tap position, costs are further increased, in particular when the number of taps is higher. The second disadvantage is the higher conduction losses of electronic switches compared to mechanical contacts.
Hence, there is a need for OLTC devices that are economically more viable, require lower maintenance, cause lower conduction losses, and provide for flexibility to meet changing regulation requirements due to the increasing share of intermittent renewable energy resources in the distribution grid.